In general, a plastic bag such as a garbage bag or a food garbage bag has a constant volume and is thrown away with a mouth thereof bound when the volume is full. However, when the volume of the plastic bag is not full, the mouth of the garbage bag is opened, garbage is placed in the garbage bag, and then the mouth thereof is bound again until the volume is full. Accordingly, when a device capable of opening a plastic bag such as a garbage bag or a food garbage bag and supporting the garbage bag or closing the garbage bag is used, the mouth of the garbage bag does not need to be opened and bound again whenever garbage is placed in the garbage bag and thus the garbage bag can be conveniently used. In addition, bad smell from the garbage can be controlled and thus the garbage bag can be used sanitarily.